Minimally invasive surgical techniques generally attempt to perform surgical procedures while minimizing damage to healthy tissue. One particular technique for achieving this goal employs flexible surgical instruments that are able reach a target work site inside a patient by at least partially following a natural lumen such as the digestive tract of the patient. Following the natural lumen allows a surgeon to operate on the work site with less need for incisions made through healthy tissue, although an incision may be needed at locations where the flexible instrument enters or leaves a natural lumen. An entry guide can be used during such a surgical procedure to facilitate insertion and removal of surgical instruments or tools during the procedure. In general, the entry guide is inserted through an incision or a natural orifice and steered along a path to a point where the distal end of the entry guide nears or reaches a target work site. The entry guide generally contains one or more instrument lumens through which different surgical instruments can be inserted or removed. This allows instruments to be changed without requiring a delicate steering procedure each time a different set of instruments is needed.
Surgical instruments and entry guides that are able to follow a natural lumen or other convoluted paths generally must be flexible, which requires these devices to have properties and abilities that are not needed in most other surgical instruments. In particular, although an entry guide must be flexible enough to navigate a convoluted path, the guide ideally should provide a stable base at the work site for manipulation of an instrument or instruments inserted through the guide. Additionally, the guide should not change shape or react to external forces in a manner that could unintentionally damage adjacent tissue. Cables or tendons may extend through all or part of an entry guide for actuation of mechanical features of the entry guide or steering of the entry guide along its path. In some advanced surgical systems, these cables are robotically operated using motors and computer aided control. (As used herein, the terms “robot” or “robotically” and the like include teleoperation or telerobotic aspects.) The forces applied through the tendons can be significant, both to overcome friction and because the lengths of entry guides and instruments can create long moment arms. A flexible surgical device needs to control these relatively large forces so that reactions or movements along the length of the device do not damage the adjacent tissue of the patient.